Present technology analog-to-digital conversion circuits have multiple input channels by using analog multiplexing circuits that allow input selection from several different external analog sources for analog-to-digital conversions thereof. However, since the absolute values of the external analog sources may not be known and have to be verified by accurate measurements, and imperfections in the analog-to-digital conversion circuitry such as offset and gain errors result in inaccurate analog-to-digital conversion measurements of the external analog voltages being measured. The offset and gain errors may result from circuit non-linearity, resistive voltage drop, and/or manufacturing process variations. Also without accurate points of reference and/or an absolute voltage reference to compare to, only relative measurements with respect to power supply voltages, e.g., VDD, are possible.